Mirrors for high power lasers require high reflectivity approaching 100%, high thermal conductivity to dissipate heat absorbed at the surface, and a tarnish free, highly polished surface. By highly polished surface, I mean a surface having relatively low rms (root mean square) roughness, i.e., rms is a statistic which numerically quantifies the variation in surface displacement or deviation from a perfect plane over a distance on the surface and which, for a highly polished reflective metallic mirror, is typically less than 15 A rms.
Currently available mirrors of the type used in laser systems are generally manufactured of materials which amount to a compromise of the basic requirements enumerated above. That is to say, currently available materials that have been used, or proposed for use, in laser mirrors fail to meet the basic requirements for reasons such as softness, loss of reflectance in alloying, susceptibility to corrosion and tarnishing, warping or distortion, expense, or complexity of manufacture.